If you are getting this error while installing Nagios Core on RaspberryPi run these commands to resolve the error and get Nagios up and running in no time. These commands wil need to be run as ROOT. You need to restart the nagios service after you carry out the changes.
Create this nagios.service file
# vi /etc/systemd/system/nagios.service
[Unit]
Description=Nagios
BindTo=network.target
[Install]
WantedBy=multi-user.target
[Service]
User=nagios
Group=nagios
Type=simple
ExecStart=/usr/local/nagios/bin/nagios /usr/local/nagios/etc/nagios.cfg
# systemctl enable /etc/systemd/system/nagios.service
# systemctl start nagios
# systemctl status nagios
After that you need to restart the Nagios Service.
# /etc/init.d/nagios restart
That’s it you should now be able to access Nagios WEBUI.